Apparatus for locking a door in a closed position are numerous and varied in design and complexity. The shortcoming of most locking apparatus is that they only operate to secure the door when the door is in a closed position. If the door should be opened to view objects on the other side or to pass small objects through the doorway, most locking apparatus will not prevent the door from being forced completely open by an unwanted intruder.
Door chains connected to the door frame and selectively connected to the door provide a means for limiting the movement of a door from a closed position. However, most door chains are limited in the amount of force exerted on a door that the chain can withstand. The chain must be connected to the door itself, which provides a minimal structural base for securing the chain and further, the force exerted on the door by an intruder is concentrated at the connection of the chain with the door. Most chains are detachably engaged by brackets connected to the door by screws or other fasteners common to the industry. The force of an intruder is concentrated on the fasteners which usually have a minimal cross-sectional area and/or a minimal securing surface in contact with the door. Screws, bolts, nails and other securing apparatus tend to strip from the door when a large force (i.e. the mass of a human body in motion) is exerted against the door. Even if the securing apparatus were to hold, it is likely that the concentration of force on such a small area of the door will break the portion of door to which the chain is attached, thus permitting the door to open.
Another method of limiting the movement of the door from a closed position is the use of door braces which are pivotally secured at a lower end to the floor and extend in angular relation to and in abutment with the door. An upper end of the door brace is received within a vertically extending slot in the door such that movement of the door from the closed position will urge the upper end of the brace to the top of the slot whereby the brace will contact a stop and thus resist further movement of the door. The problem with door braces is twofold. One, the force exerted by an intruder is still concentrated at a very small area of the door and if the door is wooden, as many doors are, the door brace could be driven through the door. Secondly, the door brace assembly extends some distance from the door and within the adjacent room, thus presenting a visually distracting sight and possibly a hazardous obstacle to an inattentive person who may inadvertently trip over the door brace.